1. Field of the Invention
The present invention concerns a method for operating a computed tomography apparatus with an x-ray radiator rotatable around a system axis, with an x-ray detector and with a radiator-side gating device that has two opposite absorber elements that can be adjusted in a straight line, in particular that can be adjusted with regard to their separation from one another, for variable delimitation of the ray beam. An examination subject is scanned during rotation of the x-ray radiator and relative translational movement between the x-ray radiator and the examination subject in the direction of the system axis.
2. Description of the Prior Art
In an examination of an examination subject or a patient in an x-ray diagnostic apparatus, the examination subject is moved into an x-ray beam emitted by an x-ray source, and the radiation actuated by the subject is detected by an x-ray detector. The examination subject is thus located in the beam path between the x-ray source and the x-ray detector. The typical x-ray tubes used as x-ray radiators emit x-ray radiation in a significantly larger solid angle than is necessary for examination at the patient. In order to prevent unnecessary radiation exposure at the patient, unnecessary x-rays are gated (blanked out). For this, in conventional x-ray apparatuses it is known to dispose a radiator-side gating device immediately after the x-ray radiator in the beam path. Such gating device is also designated as a primary beam diaphragm. For example, such a primary beam diaphragm, with diaphragm plates that can be moved opposite to one another as absorber elements, is known from European Application 0 187 245.
In computed tomography apparatuses with multi-row x-ray detectors, a detector-side beam diaphragm (or a beam diaphragm near to the detector) that is mounted in the beam path between the patient and the x-ray detector is also frequently used in addition to a radiator-side gating device that is arranged in the beam path between the x-ray radiator and the patient. It is thereby possible to shadow one or more detector rows of the multiple detector rows and to use the remaining detector rows as active detector rows.
Such a collimator is known from U.S. Pat. No. 6,396,902 the entire bearing body composed of x-ray-absorbing material must be moved. This occurs by rotation of the bearing body, which is why the bearing body is also curved around a second axis. In order to also be able to bring another diaphragm slit into the matching position, the rotation axis would have to be located at the height of the focus of the x-ray radiator. This is at best possible with very large mechanical effort.
Alternatively, the rotated bearing body would have to be readjusted into the correct position via a shifting movement, which is likewise very elaborate. Moreover, the production of a bearing body curved around two axes is expensive, because this must also be produced from x-ray-absorbing material, meaning from a material with a high atomic number.
A further disadvantage of the x-ray collimator known from U.S. Pat. No. 6,396,902 is that only a finite number of slits of discrete width can be applied or, respectively, introduced on or in the bearing body.